1. Field of the Invention
The present invention relates to a method of correcting the shape of a distorted end of a pipe and, more particularly, to a method of correcting a distorted pipe end to the original substantially true circular cross-section by applying radially inward external loads to the outer peripheral surface of the pipe end and also applying a radially outward internal pressure to the inner peripheral surface of the pipe end.
2. Description of the Prior Art
Generally, in the construction of chemical plants, power plants and the like, pipe lines interconnecting various apparatus and equipments are worked or made at the construction site by coupling or connecting pipes usually by welding. So as to obtain a good weld, it is necessary that the cross-sectional shapes of the ends of two pipes to be welded together conform each other because, if the pipe ends to be welded together have different cross-sectional shapes, the weldability is lowered to impair the mechanical strength and corrosion-resistant property of the weld, resulting in a poor reliability of the welded pipe lines.
The prior art problems will be discussed in more detail with reference to FIGS. 1 to 4.
FIG. 1 shows two straight pipe elements 1 and 2 to be welded together. The pipe element 1 has previously been connected with a branch pipe 3 by welding 5. The end 4 of the pipe element 1 to be welded to the pipe element 2 has been deformed or distorted to an oval cross-section as shown in FIG. 2A due to the thermal deformation caused by the heat input. On the other hand, the end of the other pipe element 2 to be welded to the pipe end 4 of the pipe element 1 presents a substantially true circular cross-section because no branch pipe has been welded to the pipe element 2. If the pipe element 1 is welded at its end 4 to the circular end of the pipe element 2 simply by a single-side welding effected from the outside only, it is impossible to obtain a good weld joint 5a having a good penetration bead 6 as shown in FIG. 3.
To avoid this problem, the shape of the distorted end 4 of the pipe element 1 is corrected to the original circular shape before the pipe end is welded to a mating pipe element. FIG. 4 shows an example of the conventional method of correcting distorted pipe ends. According to this method, a plurality of L-shaped legs 7 are welded to the outer peripheral surface of a pipe element 1 at a suitable circumferential pitch. A correction bolt 8 is screwed into free end of each leg 7. As these correction bolts 8 are screwed, the inner ends of these bolts abut and press the end 4 of the pipe element 1 radially inwardly. The distorted pipe end 4 of the pipe element 1 is thus corrected by the application of radial loads P to resume the original true circular cross-section. The legs 7 are removed after the completion of the correction. This typical conventional correction method, however, cannot provide a uniform correction over the entire periphery of the pipe end because the radial loads P are concentrated to circumferentially spaced points on the pipe end. In addition, since the legs 7 are secured to the pipe element 1 by welding, the residual welding stress remains even after the removal of the legs to adversely affect the corrosion-resistant and other properties of the pipes. It is also to be pointed out that this conventional correction method takes an impractically long time.